This invention relates to an optical CVD (chemical vapor deposition) method for use in depositing a thin film on a substrate as a CVD film and to an optical CVD device for use in carrying out the method. The thin film has usually a thickness of the order of a micron in the manner known in the art.
Various optical CVD methods are extensively studied and developed in recent years as processes for manufacturing semiconductor devices. For example, the optical CVD method is used in forming electrodes and wirings on integrated circuits, forming photolithographic masks on wafers, and doping wafers with impurities. This is because the optical CVD method lowers the temperature of the process and shortens the progress of work.
On carrying out the optical CVD method, a CVD gas or vapor is photo-induced, namely, subjected to photochemical reaction. The CVD gas comprises a material which should be deposited as the CVD film on a predetermined area of a substrate. A pulsed ultraviolet beam is used as an optical beam in subjecting the CVD gas to the photochemical reaction through an optical system. The pulsed ultraviolet beam is preferred because of the capability of heating the substrate to a raised temperature with the temperature rise locallized to the predetermined area either when the substrate is thermally insulating or when the predetermined area is an area of thermally insulative layer formed on the substrate. The pulsed ultraviolet beam is consequently capable of forming a fine pattern directly on the substrate when given pertinent characteristics. For use in generating such a pulsed ultraviolet beam, an optical source may be an excimer laser or a fourth harmonic producing system comprising an Nd:YAG laser.
A paper was contributed by H. Yokoyama et al of NEC Corporation, the present assignee, to "Technical Digest" of "CLEO '84" (Conference on Lasers and Electro-Optics held June 19 to 22, 1984), pages 222 to 223 and 225, as Paper No. FD6 of the title of "Importance of Photo-induced Surface Heating for Film Quality Improvement in Excimer Laser-Induced Metal CVD." According to the Yokoyama et al paper, a KrF excimer laser is used as an optical source of the pulsed ultraviolet beam. With chromium hexacarbonyl vapor used as the CVD gas, a chromium film is deposited as the CVD film on a glass substrate. It is clarified that a photothermal or photo-heating effect is important in achieving a smooth film surface and a tenacious adhesion strength.
A paper was submitted by Yukio Morishige, the present applicant, et al in the Japanese language together with a summary in English to "Proceedings of the 27th Symposium on Semiconductors and Integrated Circuits Technology" of Electronic Materials Committee of the Electrochemical Society of Japan, held in Tokyo on Dec. 4 and 5, 1984, pages 6 to 11, under the title of "Micrometer-Scale Cr Film CVD with kHz Repetition Rate UV Light Pulses" according to the summary. In the Morishige et al paper, a kHz repetition fourth harmonic ultraviolet beam of a Q-switched Nd:YAG laser is used as the optical beam in depositing a chromium film on a quartz substrate from a CVD gas of chromium hexacarbonyl vapor. It has been demonstrated that the chromium film broadens only about one micron from the predetermined area of 12 microns square and that the CVD method is very attractive for an application in repairing a photomask pinhole defect. These facts are further proved by simulation for broadening of the area which is subjected to the photothermal effect. Incidentally, an optical attenuator was used in an experimental setup in studying relationships between an optical intensity of the optical beam at the predetermined area and the film quality which the chromium film has at its surface.
An article was contributed by H. Yokoyama, named above, et al to Applied Physics, Volume A 37 (1985), pages 25 to 30, under the title of "Photothermal Effect Contribution on Film Quality Improvement in Excimer-Laser Induced Metal CVD." A KrF excimer laser is used as the optical source of the pulsed ultraviolet beam in depositing a chromium film on a quartz substrate with chromium hexacarbonyl vapor used as the CVD gas. A variable attenuator was used in making the optical beam have various optical intensities to study the effects which the optical intensity causes on surface morphology and adhesion.
In the above-referenced papers and article, the substrate is held in a reaction or CVD cell. The CVD gas is supplied through the reaction cell by a gas supplying system. The pulsed optical beam is given a constant average intensity, which is selected to attain an optimum irradiating condition. It is already known that such a CVD film is depthwise heterogeneous in composition and is not sufficiently strongly adhesive to the substrate.
An optical CVD method is revealed in Japanese Patent Prepublication (Publication of Unexamined Patent Application) No. 197,560 of 1984 for an application filed by NEC Corporation, assignee from Hiroyuki Yokoyama, the sole inventor. According to the method, the adhesion is strengthened by irradiating the predetermined area of the substrate in vacuum by a very strong laser beam immediately before deposition of the CVD film. An expensive optical CVD device is, however, indispensable on carrying out the method when the substrate is of quartz or of a like material which is transparent to electromagnetic waves of a wide wavelength region. This is because the laser beam must have a limited wavelength which can effectively be absorbed by the substrate. The optical source and the optical system therefore becomes expensive.
Another optical CVD method is disclosed in Japanese Patent Prepublication No. 208,065 of 1984 for an application filed by NEC Corporation, assignee from Hiroyuki Yokoyama, the inventor. According to the method, the adhesion is intensified by irradiating the substrate with a laser beam superposed on the pulsed ultraviolet beam. The laser beam should have a wavelength in the visible or an infrared wavelength region and a sufficiently high intensity within an intensity range in which the laser beam would not subject the CVD gas to thermal dissociation. It has, however, been confirmed that the adhesion is not much improved. More particularly, the CVD film readily peals off when subjected to a scratch test by a pair of tweezers. This is because a restriction is unavoidably imposed on the temperature rise so that the substrate surface is only insufficiently cleaned.
Still another optical CVD method is revealed in Japanese Patent Prepublication No. 209,642 of 1984 for an application filed by NEC Corporation, assignee from Kunihiko Washio, the inventor. The adhesion is augmented by surface etching the substrate by gas discharge in an inactive gas. Devices are therefore necessary for the gas discharge. This renders the optical CVD device bulky and expensive.
Yet another optical CVD method is disclosed in Japanese Patent Prepublication No. 53,015 of 1985 for an application filed by NEC Corporation, assignee from Hiroyuki Yokoyama, the inventor. The adhesion is reinforced by irradiating the CVD film with a laser beam after removal of the CVD gas from the reaction cell. It appears according to Yokoyama that the CVD film is physically adsorbed by the substrate when deposited and that a chemical bond occurs between the CVD film and the substrate during irradiation by the laser beam. It has, however, been found that hydrocarbons and other impurities are inevitably included in the CVD film during an initial period of deposition and that such impurities would vaporize during the laser beam irradiation to weaken the CVD film.
It would be worthwhile to point out here the fact that the methods proposed in the above-referenced patent prepublications are incapable of rendering the composition of the CVD film depthwise homogeneous. The adhesion is strengthened, if possible, either by an expensive optical CVD device or by accompanying defects. Furthermore, the adhesion depends on degrees to which the substrate is cleaned at least at its surface before subjected to the optical CVD method. In other words, the conventional optical CVD methods are defective in reproducibility and reliability.